Capillary reactor distribution device and method

ABSTRACT

A capillary reactor distribution device comprising first and second capillary pathways ( 2, 3 ) which meet at a junction ( 5 ) and a third capillary pathway ( 4 ) which leads away from the junction ( 5 ), the capillary pathways ( 2, 3, 4 ) being dimensioned such that, when first and second immiscible fluids ( 14, 15 ) are fed along respectively the first and second capillary pathways ( 2, 3 ) under predetermined laminar flow conditions, the first and second fluids ( 14, 15 ) chop each other into discrete slugs ( 16, 17 ) which pass along the third capillary pathway ( 4 ). Molecular mixing between the fluids ( 14, 15 ) takes place by way of axial diffusion between adjacent slugs ( 16, 17 ) and by way of internal circulation within each slug ( 16, 17 ) as the slugs ( 16, 17 ) progress along the third capillary pathway ( 4 ).

The present invention relates to a capillary reactor distribution deviceand method and in particular to a distribution device and method forinterfacing at least two immiscible fluids.

Improved methods of manufacturing at microscales have opened up newopportunities for the development of compact, efficient and highlycontrollable reactors. Rapid mass and heat transfer between fluids maynow be engineered into reactors by the use of small dimensions for fluidtransport. Short path lengths for thermal and molecular diffusion canprovide an ideal environment for rapid exothermic/endothermic reactionswhile maintaining a laminar flow regime. Scale requirements forefficient mixing by diffusion can be calculated using the followingequation (Crank, J., 1975, “The Mathematics of Diffusion”, 2nd edition,Clarendon Press, Oxford):d=(Fo.Dt)^(0.5)  (1)where D is the diffusivity of the reacting molecules in the fluid, t isthe residence time and Fo the Fourier number determining the level ofmixing. For most systems, Fo=1 would be chosen. Reactions that maybenefit most from this technique are those where phases cannot mix toform a single phase, such as liquid-gas, liquid-solid, gas-solid andimmiscible liquid flow.

Two general methods are available for efficiently contacting twoimmiscible liquid streams within a microreactor. The first is the use ofparallel liquid streams as described in WO 97/39814 and WO 99/22858where diffusion is perpendicular to the flow direction. The second isthe use of dispersed/continuous phase flow where one phase is in theform of small droplets within the other phase or slug flow where eachphase is in the form of a series of slugs. Diffusive mass transfer maybe aided by internal circulation within the droplets generated by theshear flow as demonstrated by Clift, R., Grace, J. R. and Weber, M. E.(1978, “Bubbles, drops and particles”, Academic Press, New York).

Several benefits and drawbacks accompany these two techniques. In theuse of parallel flow, it is difficult to achieve stability and similarresidence times for liquids of significantly different viscosity or flowrate. On the other hand, for droplet flow, velocities of the dispersedand continuous phases remain similar, and a wider range of flow rateratios can be tolerated. However, parallel flow has the advantage ofeasy bulk separation of the liquids after reaction, whereas droplets andslugs need to be separated by way of centrifugal or gravitationalaction.

Typical diffusion rates within liquids are in the range 10⁻⁸m²s⁻¹ to10⁻⁹m²s⁻¹, and therefore, from equation (1) above, length scales fordiffusion within the reactor are required to be of the order of 100 μmfor rapid reactions requiring 1 to 10 s residence time. However, masstransfer enhancement due to internal vortices in droplet/slug flowaugment this process and allow larger channel dimensions to be usedwhile maintaining fast reaction times.

It is known from U.S. Pat. No. 5,921,678 to provide a microfluidicsub-millisecond mixer in which two capillary pathways meet head-on andin which a third capillary pathway leads away from the junction of thefirst two, thereby forming a T-junction. Two miscible fluids are thendirected along the first two capillary pathways so as to meet at thejunction, mix in turbulent conditions, and then flow along the thirdcapillary pathway where reaction takes place. The third capillarypathway is very short, so as to constrain reaction time tosub-millisecond timescales, before the reactants are quenched and thenseparated. This prior art mixer is not suited for use with immisciblefluids.

According to a first aspect of the present invention, there is provideda capillary reactor distribution device comprising first and secondcapillary pathways which meet at a junction and a third capillarypathway which leads away from the junction, the capillary pathways beingdimensioned such that, when first and second immiscible fluids are fedalong respectively the first and second capillary pathways underpredetermined laminar flow conditions, the first and second fluids chopeach other into discrete slugs which pass along the third capillarypathway.

According to a second aspect of the present invention, there is provideda method for contacting two immiscible fluids, wherein a first fluid isfed under laminar flow conditions along a first capillary pathway and asecond fluid is fed under laminar flow conditions along a secondcapillary pathway, the first and second capillary pathways meeting at ajunction having a third capillary pathway leading away therefrom, andwherein the flow conditions in each of the first and second capillarypathways are selected such that the first and second fluids chop eachother into discrete slugs which pass along the third capillary pathway.

In general, the two immiscible fluids are both in the liquid phase,although liquid/gas, solid/liquid/liquid and solid/liquid/gas reactionsmay be performed in the device and method of the present invention. Inthe case of solid/liquid/liquid and solid/liquid/gas reactions, thesolid phase may be coated on a surface of the third capillary pathway,for example in the form of a catalyst coating for liquid/liquid orliquid/gas reactions.

In preferred embodiments, the first and second capillary pathways meetsubstantially head-on at the junction, although for some applicationsthe first and second capillary pathways may be arranged to meet atangles other than substantially 180 degrees. For example, the first andsecond capillary pathways may be arranged to meet at an angle ofsubstantially 90 degrees, at an angle between 90 and 180 degrees, or atan angle between 0 and 90 degrees. In some embodiments, the first andsecond capillary pathways may be arranged to meet at an angle of 90degrees to 300 degrees, and the third capillary pathway may liesubstantially midway between the first and second capillary pathways orat substantially 180 degrees to the first capillary pathway.

Advantageously, the capillary pathways are formed in or lined with anon-stick or low surface energy material, such as a fluoropolymer (e.g.PTFE or PVDF).

It is believed that the mechanism whereby the first fluid chops thesecond fluid into discrete slugs is as follows. Assuming that each fluidapproaches the junction at a constant flow rate, and considering thecase where the first and second capillary pathways meet substantiallyhead-on, the first fluid flows preferentially into the third, exitcapillary pathway while the second fluid forms an interface at thejunction. The interface is moved into the third capillary pathway by thedriving pressure from the first fluid supply aided by viscous shear fromthe second fluid. When the interface grows to a size that blocks thefirst fluid from entering the third pathway, the process switches andthe second fluid flows preferentially into the third pathway while thefirst fluid forms an interface, which then moves into the third pathwaybefore the process switches back again. This alternating flow of thefirst and second fluids generates a series of slugs in the thirdpathway. The lengths of the slugs is believed to be most significantlygoverned by the ratios of the widths of the inlet and outlet capillarypathways, the surface energy of the walls of the capillary pathways andthe ratio of the first and second fluid flow rates. In particular, theratio of slug lengths is generally substantially the same as the ratioof fluid flow rates. The lengths of the slugs are governed also to alesser extent by the total fluid flow and the viscosities of the firstand second fluids, and also by interfacial phenomena. Production ofslugs is most preferably achieved in materials which do not have verylow contact angles with either of the fluids.

As the slugs progress along the third capillary pathway, mixing of thefirst and second fluids on a molecular level is achieved by both axialdiffusion between adjoining slugs and also by internal circulationwithin each slug, the latter process generally being the dominant one.Both forms of mixing will generally increase as slug length is reduced.For rapid mixing, the smallest slug length should be of the order of thewidth of the third capillary pathway, and the longest slug length notgreater than 100 times the width thereof, and preferably not greaterthan 10 times the width thereof. It is particularly preferred that thelongest slug length is not greater than twice the width of the thirdcapillary pathway.

The device of the present invention may comprise a solid block of anyappropriate material having the capillary pathways bored thereinto.Fluid may be pumped to the block and removed therefrom by way ofstandard capillary tubes which are connected to the capillary pathwaysbored into the block. The capillary tubes may connect to the capillarypathways at or near external surfaces of the block, or may connectthereto within the body of the block, near to the junction.Alternatively, passages may be bored into the block so as snugly toreceive the capillary tubes, the junction being defined by the ends ofthe capillary tubes themselves where they meet within the block.Preferably, O-ring or similar seals are provided where the capillarytubes enter the block so as to prevent pressure losses as fluid ispumped towards the junction. Advantageously, the internal volume of thecapillary pathways within the block is kept as small as possible.

Typical flow rates through the device of the present invention rangefrom 10 nls⁻¹ to 100 μs⁻¹, with preferred flow rates ranging from 100nls⁻¹ to 10 μls⁻¹. Flow rate ratios between the fluids in the first andsecond capillary pathways are advantageously not greater than 10:1 andare preferably close to unity for high mixing efficiency.

Alternatively, the device of the present invention may be formed by atleast two generally laminar plates mounted one directly on top of theother such that a surface of one plate contacts a surface of the otherplate, at least one of the surfaces being provided with features servingto define the capillary pathways. The plates will generally be inregistration with each other. The at least one surface may includechannels or ridge-like protrusions or both, such that when the surfacesof the plates are contacted, the required capillary pathways are definedbetween the plates. The channels and/or the protrusions may be formed byan etching process, or may be micromachined or moulded. Further plateswith suitable surface features may be stacked on top of the at least twoplates so as to produce a multi-layer device.

Either the first or the second capillary pathway or both may be providedwith a fluid filter to help prevent stray particles from entering thedevice and which may block the capillary pathways. In the solid blockembodiment of the present invention, the fluid filter is preferablylocated between either one or both of the input capillary tubes and thefirst or second capillary pathways.

The device and method of the present invention is particularly usefulfor conducting reactions between organic and aqueous liquids, forexample the nitration of benzene and toluene as discussed hereinafter.Other applications include rapid mass transfer for liquid-liquidextraction and small volume reaction testing for analytical purposes.

For a better understanding of the present invention and to show how itmay be carried into effect, reference shall now be made by way ofexample to the accompanying drawings, in which:

FIGS. 1 to 3 show three alternative configurations of the distributiondevice of the present invention;

FIGS. 4 to 7 show a proposed mechanism for slug formation within thedevice of the present invention;

FIG. 8 shows a series of liquid slugs within a capillary tube;

FIG. 9 shows a capillary reactor including a device of the presentinvention;

FIG. 10 is a graph of reaction rates against temperature for benzenenitration in the reactor of FIG. 9;

FIG. 11 is a graph of reaction rates for benzene nitration in thereactor of FIG. 9 at different flow velocities;

FIG. 12 is a graph of nitrotoluene concentration against acid/organicflow ratio for toluene nitration along different reaction pathwaylengths in the reactor of FIG. 9;

FIG. 13 is a graph of transfer times for acetic acid from organic toaqueous phase in a 0.38 mm×0.38 mm glass channel;

FIG. 14 shows an alternative embodiment of the device of the presentinvention; and

FIG. 15 shows the flow dynamics of slug propagation along a capillarypathway.

FIG. 1 is a section through a PTFE block 1 into which first, second andthird tubular capillary pathways 2, 3, 4 of diameter 0.5 mm and length 5mm have been bored. The first and second capillary pathways 2, 3 meethead-on at a junction 5, and the third capillary pathway 4 leads awayfrom the junction 5 substantially at right angles to the first andsecond capillary pathways 2, 3. Boreholes 6, 7, 8 are provided so as toallow PTFE capillary tubes 9, 10, 11 with internal diameters of 0.15 mmto be snugly inserted into the block 1 and to connect respectively tothe capillary pathways 2, 3, 4. Each capillary tube 9, 10, 11 isprovided with an O-ring seal 12 where it enters the block 1 so as toreduce pressure losses within the block 1, and filters 13 are providedwhere the feed capillary tubes 9, 10 connect with the first and secondcapillary pathways 2, 3.

An alternative embodiment is shown in FIG. 2, where boreholes 6, 7, 8extend to a middle portion of the block 1 so as to allow the capillarytubes 9, 10, 11 with internal diameters of 0.15 mm to meet and form thejunction 5.

A further alternative embodiment is shown in FIG. 3, where the capillarypathways 2, 3, 4 of diameter 0.8 mm extend from the junction 5 toexternal surfaces of the block 1, where the capillary tubes 9, 10, 11are connected.

It is to be noted that in some embodiments, the positions of capillarypathway 3 and capillary tube 10 may be swapped with those of capillarypathway 4 and capillary tube 11, such that the two feed capillarypathways 2, 3 meet substantially at right angles.

By using a syringe driver (not shown) to inject dyed kerosene 14 alongcapillary tube 9 and thence capillary pathway 2, and water 15 alongcapillary tube 10 and thence capillary pathway 3, a series of slugs 16,17 were formed in capillary pathway 4 and thence capillary tube 11, asshown in FIGS. 4 to 7 and 8.

The mechanism for slug 16, 17 formation is shown in FIGS. 4 to 7. InFIG. 4, water 15 flows preferentially from the capillary pathway 3 intothe capillary pathway 4, while kerosene 14 forms an interface 18 at thejunction 5. Due to the driving pressure behind the kerosene 14, theinterface 18 is moved into the junction 5 and towards the capillarypathway 4, aided by viscous shear from the water 15 as shown in FIGS. 5and 6. When the interface 18 has completely moved into the capillarypathway 4, as shown in FIG. 7, the flow of water 15 is blocked andkerosene 14 then flows preferentially into the capillary pathway 4, withthe water 15 forming an interface 18′. The process is then reverseduntil the interface 18′ moves into the capillary pathway 4 and water 15again flows preferentially into the capillary pathway 4. The alternatingmovement of the interface 18, 18′ causes a series of kerosene slugs 16and water slugs 17 to be formed in the capillary pathway 4 and thencethe capillary tube 11 as shown in FIG. 8.

Flow rates of 0.8 to 13 μls⁻¹ were tested, with aqueous/organic flowratios of 2:1 and 1:1. The embodiment of FIG. 1 was found to produceslug 16, 17 lengths of 2.1 to 5.5 mm, that of FIG. 2 to produce lengthsof 0.3 to 0.9 mm and that of FIG. 3 to produce lengths of 18 to 30 mm.This indicates that the low internal flow volume of the FIG. 2embodiment helps to produce short slug 16, 17 lengths.

FIG. 9 shows a reactor comprising an aqueous phase pump 19 and anorganic phase pump 20, respectively connected to capillary tubes 9, 10which then pass into a distribution device 1 of the type shown in FIG.2. An output capillary tube 11 passes from the device 1 and through aheater 21, inside which the capillary tube 11 is coiled for efficientuse of space. The capillary tube 11 then passes from the heater 21 to acollection bottle 22. In the following examples, an aqueous phasereactant was pumped by pump 19 along capillary tube 9 and an organicphase reactant by pump 20 along capillary tube 10. Slugs (not shown) ofaqueous phase and organic phase reactant were formed in the capillarytube 11 by the device 1, and then passed along the capillary tube 11,through the heater 21 and thence to the collection bottle 22 whichcontained solvents to halt the reaction between the reactants and todilute the organic phase reactant. Analysis of the organic conversionsdiscussed in the following examples was performed using gaschromatography.

EXAMPLE 1 Benzene Nitration

Distributors having capillary tubes made out of 316 stainless steel withrespectively 127 μm, 178 μm and 254 μm bore sizes were constructed. Asyringe driver was used to supply the liquids for the reaction and aheating, bath was used to control the reactor temperature. The nitrationreaction involved contacting a stream of benzene with a stream of nitricand concentrated sulphuric acids. Various acid strengths and reactortemperatures were used in the nitration work and comparisons made of thereaction rate and by-product formation. A shell reaction model was usedin calculating the reaction rate for the process. This assumes thatnitration takes place in a acid boundary layer surrounding the organicphase. For this model mass transfer into the region and kinetic reactionrate within the region are equally important in the overall observedrate. The resulting equation governing this process can be written as a1.5^(th) order reaction, as shown in equation (2), where X is theproportion of the initial nitric acid remaining at time t.$\begin{matrix}{\frac{\mathbb{d}X}{\mathbb{d}t} = {- {CX}^{1.5}}} & (2)\end{matrix}$

The value of constant C is determined by the mass transfer rate into thereaction zone and kinetic reaction rate within the zone. Integration ofequation (2) yields the following equation for nitric acid concentrationat time t. $\begin{matrix}{X = \left( {1 + \frac{Ct}{2}} \right)^{- 2}} & (3)\end{matrix}$

One method of characterising the nitration process is by comparison ofthe indicated initial reaction rates. This is defined as the reactionrate at the start of the process and can be calculated from equations(2) and (3) as, $\begin{matrix}{{{{InitialRate} = \frac{\mathbb{d}X}{\mathbb{d}t}}}_{t = 0} = {C = \frac{2\left( {X^{{- 1}/2} - 1} \right)}{t}}} & (4)\end{matrix}$where X is the measured value at time t. A similar formula can beobtained for the organic reaction rate by substitution of X with theproportion of non-nitrated organic remaining. A comparison of thereaction rate observed for 127 μm and 254 μm bore capillary tubing undersimilar conditions is shown in FIG. 10. A high sulphuric acidconcentration was used to ensure fast nitration kinetics and promote amass transfer limited regime. Comparing the results for the twocapillary diameters clearly shows enhanced performance at the smallerscale implying improved mixing.

FIG. 11 illustrates the influence of capillary flow rate on organicreaction rate. An enhancement in reaction rate was observed when fasterflow was applied to the reactor especially for the conditions with thefastest kinetics. This would indicate that increased velocity wasleading to increased mixing. The primary source of this improvement ismost likely due to increased internal circulation within the liquids,although some variation in slug length may also be contributing.

EXAMPLE 2 Toluene Nitration

Recent work has examined the nitration of toluene using a PTFE capillaryreactor. The use of PTFE gave a more corrosion resistant system withless chance of blockage. Blockages were found to occur occasionally inthe stainless steel system between runs probably due to sulphuric acidcorrosion. However, no such problems occurred with the PTFE basedsystem. Two HPLC pumps were used to supply the flow to the reactor witha greater run time capability.

Toluene nitration was performed using 150 μm bore tubing using a rangeof acid strengths and reactor temperatures. Results showed a lowerinfluence of temperature on reaction rate than benzene nitration whentemperatures of greater than 75° C. were used. Typical nitric acidreaction rates for the system are shown in Table 1 for a range of acidand organic flow ratios. Observed rates were generally higher than forbenzene under similar conditions.

TABLE 1 Initial nitric acid reaction orates for toluene nitration in a150 μm PTFE reactor. (Experiments used 78% H₂SO₄ with 7% HNO₃)Acid:Organic Reaction Rate Reaction Rate Flow Ratio at 25° C. (min⁻¹) at60° C. (min⁻1) 2:1 3.27 6.10 3:1 2.81 6.12 5:1 2.25 4.39 7:1 1.65 4.17

FIG. 12 shows nitrotoluene production for a range of flow ratios andreactor lengths. The results show an increasing production ofnitrotoluene when larger ratios of acid to organic were used in thereactor. However, the little improvement in conversion is observed forflow ratios exceeding 5:1 even though more acid is available fornitration. This is also reflected in the lower reaction rates shown inTable 1 for the higher flow ratios. This would suggest a poorer mixingenvironment for the high flow ratios probably due to increased acid sluglength.

End effects from possible post reactor nitration were also examined forboth benzene and toluene nitration. Output from reactor tubes ofdifferent lengths were compared to check that increased length providedhigher conversion implying that the reaction was taking place within thecapillary tube and not within the sampling system. FIG. 12 shows theresults from three different reactor lengths using the same conditionsand shows in general that higher conversion was achieved for the longertubes.

Visual analysis of liquid-liquid flow through a capillary reactor hasshown that a pattern of alternating organic/aqueous slugs can beproduced each having lengths down to 300 μm. The work has also shown theimportance of low internal volumes in distributor design for controllingthe pattern of liquid-liquid flow produced.

Reaction results for benzene and toluene nitration have indicatedreaction rates in the range of 1 to 8 min⁻¹ can be produced from acapillary reactor. This would indicate residence times for completeconversion to be in the region of 10 to 60 seconds. A comparison withsome existing benzene nitration processes (as described in the indicatedU.S. patents) is shown in Table 2. This illustrates that even with 178μm bore tubing the capillary reactor process is competitive.

TABLE 2 Comparison of benzene nitration performance with existingprocesses Information Inlet Outlet H₂SO₄ Conversion By-product Time Ratesource (° C.) (° C.) (mass %) (%) (ppm) (s) (min⁻¹⁾ US 4,091,042 80 12860.6 89.5 1000 120 0.9 US 4,091,042 80 134 65.2 99.1 2090 120 2.1 US5,313,009 95 120 69.5 90 1750 25 4.6 Capillary 178 μm 90 90 77.7 94.04600 24.4 5.90 Capillary 178 μm 90 90 72.2 60.7 Below 26.1 1.6 1000

Ultimately, narrow channel microreactors based on this technique ofliquid-liquid contacting will require shorter path lengths for diffusionto improve efficiency and lower by-product production. The use ofmicrofabricated devices with more sophisticated distribution will berequired to chop the liquids into smaller slugs or droplets. Scale-up ofthe devices for chemical production will be achieved through use ofparallel channels whilst their use for analysis will be facilitatedthrough small on-chip versions.

Referring now to FIG. 13, there is shown an alternative embodiment ofthe present invention comprising two laminar plates 40, 41 which aremountable one 40 on top of the other 41 such that the plates 40, 41 arein registration with each other. The plates 40, 41 are made out of anon-stick material such as PTFE, and an upper surface 42 of plate 40 isprovided with etched channels defining capillary pathways 43, 44, 45which meet at a junction 46. Input capillary pathways 43, 44 meetsubstantially head-on at the junction 46, and output capillary pathway45 leads away therefrom substantially at right angles to the capillarypathways 43, 44. The other plate 41, when mounted on top of plate 40,provides a top surface for the capillary pathways 43, 44, 45. The plate41 may be secured to the plate 40 by way of welding, adhesives,mechanical clamps or other suitable means.

EXAMPLE 3 Acetic Acid Titration

Mass transfer performance of a glass capillary reactor distributiondevice having capillary pathway channels 0.38 mm wide and 0.38 mm deepin a standard configuration, as shown for example in FIG. 13, wasexamined using a titration reaction. Kerosene loaded with 0.65 moles perliter of acetic acid was used in conjunction with an aqueous solution ofsodium hydroxide at various concentrations of NaOH and containing phenolred pH indicator. Equal flow rates of each liquid phase were fed throughthe glass device and the time taken to transfer different quantities ofacetic acid, as indicated by colour change in the aqueous system, wasmeasured at two different flow velocities. Typical slug lengths producedwere 1.1 mm to 1.6 mm long. The results are shown in FIG. 14. These showa significant enhancement in performance gained by vortex mixing insidethe slugs, compared with that expected from pure diffusion at thisscale, and also that higher flow velocity produces faster mass transfer.

Finally, FIG. 15 illustrates the flow dynamics of a pair of slugs 16, 17passing along a capillary pathway 45 of a device as shown in FIG. 14.The general flow direction is indicated by arrow A, with the internalcirculation patterns being shown for each slug 16, 17, these patternsbeing caused by friction between the walls of the pathway 45 andboundary layers of the slugs 16, 17 as they progress along the pathway45. Inter-slug diffusion occurs at an interface 50 between the slugs 16,17.

Nomenclature C Reaction rate constant s⁻¹ D Diffusivity m² · s⁻¹ d Pathlength for diffusion m Fo Fourier number — t Residence time s XProportion of nitric acid remaining —

1. A method for contacting two immiscible fluids, wherein a first fluidis fed under laminar flow conditions along a first capillary pathway anda second fluid is fed under laminar flow conditions along a secondcapillary pathway, the first and second capillary pathways meeting at ajunction having a third capillary pathway leading away therefrom, andwherein the flow conditions in each of the first and second capillarypathways are selected such that the first and second fluids are fedcontinuously and simultaneously alone the first and second capillarypathways, the first and second fluids chop each other into discreteslugs which pass along the third capillary pathway.
 2. A methodaccording to claim 1, wherein the first and second capillary pathwaysmeet substantially head on.
 3. A method according to claim 1, whereinthe first and second capillary pathways meet substantially at rightangles.
 4. A method according to claim 1, wherein the first and secondcapillary pathways meet at an angle between 90 and 180 degrees.
 5. Amethod according to claim 1, wherein the first and second capillarypathways meet at an angle between 0 and 90 degrees.
 6. A methodaccording to claim 1, wherein the first and second capillary pathwaysmeet at an angle from 90 to 300 degrees.
 7. A method according to claim1, wherein the fluids are both liquids.
 8. A capillary reactordistribution device comprising first and second capillary pathways whichmeet at a junction and a third capillary pathway which leads away fromthe junction, an aqueous phase pump operatively connected to the firstcapillary pathway, an organic chase pump operatively connected to thesecond capillary pathway, the capillary pathways being dimensioned suchthat, when first and second immiscible fluids are fed along respectivelythe first and second capillary pathways by the respective pumps atflowrates between about 10 nl/s to 100 μl/s, the first and second fluidschop each other into discrete slugs which pass along the third capillarypathway.
 9. A device as claimed in claim 8, wherein the first and secondcapillary pathways meet substantially head on.
 10. A device as claimedin claim 9, comprising at least two generally laminar plates mounted onedirectly on top of the other such that a surface of one plate contacts asurface of the other plate, at least one of the surfaces being providedwith features serving to define the capillary pathways.
 11. A device asclaimed in claim 8, wherein the first and second capillary pathways meetsubstantially at right angles.
 12. A device as claimed in claim 8,wherein the first and second capillary pathways meet at an angle between90 and 180 degrees.
 13. A device as claimed in claim 8, wherein thefirst and second capillary pathways meet at an angle between 0 and 90degrees.
 14. A device as claimed in claim 8, wherein the first andsecond capillary pathways meet at an angle from 90 to 300 degrees.
 15. Adevice as claimed in any preceding claim, comprising a solid block intowhich the capillary pathways have been bored or otherwise formed.
 16. Adevice as claimed in claim 15, wherein the solid block is made out of anon-stick material.
 17. A device as claimed in claim 16, wherein thesolid block is made out of a material having a low surface energy.
 18. Adevice as claimed in claim 16, wherein the capillary pathways extendfrom an interior portion of the solid block towards an outer surfacethereof, and wherein attachment means are provided for attachingexternal capillary tubes to the capillary pathways.
 19. A device asclaimed in claim 15, wherein the solid block is made out of a materialhaving a low surface energy.
 20. A device as claimed in claim 19,wherein the capillary pathways extend from an interior portion of thesolid block towards an outer surface thereof, and wherein attachmentmeans are provided for attaching external capillary tubes to thecapillary pathways.
 21. A device as claimed in claim 15, wherein thecapillary pathways are lined with a non-stick material.
 22. A device asclaimed in claim 21, wherein the capillary pathways are lined with amaterial having a low surface energy.
 23. A device as claimed in claim21, wherein the capillary pathways extend from an interior portion ofthe solid block towards an outer surface thereof, and wherein attachmentmeans are provided for attaching external capillary tubes to thecapillary pathways.
 24. A device as claimed in claim 15, wherein thecapillary pathways are lined with a material having a low surfaceenergy.
 25. A device as claimed in claim 24, wherein the capillarypathways extend from an interior portion of the sold block towards anouter surface thereof, and wherein attachment means are provided forattaching external capillary tubes to the capillary pathways.
 26. Adevice as claimed in claim 15, wherein the capillary pathways extendfrom an interior portion of the solid block towards an outer surfacethereof, and wherein attachment means are provided for attachingexternal capillary tubes to the capillary pathways.
 27. A device asclaimed in claim 26, wherein the attachment means are located on anouter surface of the solid body.
 28. A device as claimed in claim 26,wherein the attachment means are located or extend within the solidblock.
 29. A device as claimed in claim 26, wherein the attachment meansinclude O-ring seals.
 30. A device as claimed in claim 29, wherein thecapillary pathways are defined by capillary tubes inserted intoboreholes provided in the solid block and positioned so that mutuallyabutting ends of the capillary tubes form the junction.
 31. A device asclaimed in claim 28, wherein the capillary pathways are defined bycapillary tubes inserted into boreholes provided in the solid block andpositioned so that mutually abutting ends of the capillary tubes formthe junction.
 32. A device as claimed in claim 8, comprising at leasttwo generally laminar plates mounted one directly on top of the othersuch that a surface of one plate contacts a surface of the other plate,at least one of the surfaces being provided with features serving todefine the capillary pathways.
 33. A device as claimed in claim 32,wherein the surface features comprise channels.
 34. A device as claimedin claim 32, wherein the surface features comprise ridge-likeprotrusions.
 35. A device as claimed in claim 32, wherein the plates aremade of a non-stick material.
 36. A device as claimed in claim 32,wherein the plates are made of a material having a low surface energy.37. A device as claimed in claim 32, wherein the contacting surfaces ofthe plates are coated with a non-stick material.
 38. A device as claimedin claim 32, wherein the contacting surfaces of the plates are coatedwith a material having a low surface energy.
 39. A device as claimed inclaim 8, wherein at least one of the first and the second capillarypathways is provided with a filter.
 40. A device as claimed in claim 8,wherein at least one of the first, second or third capillary pathways islined with a chemically reactive material.
 41. A device as claimed inclaim 40, wherein the material is a solid heterogeneous catalyst.